1. Field of the Invention
This invention relates to a method of manufacturing a hollow fiber, and more particularly to a method of manufacturing a hollow fiber in which a spinning solution of cellulose ester dissolved in an organic solvent is extruded from an annular slit, while simultaneously introducing a core liquid into the inside hollow portion of the extruded spinning solution.
A hollow fiber made of cellulose ester, particularly cellulose acetate or of cellulose as hydrolyzate of cellulose ester is widely used as a membrane having superior selective permeabilities. Required performances of hollow fibers for separation of substances differs variously according to their intended use. For example, in the case that hollow fibers are used for reverse-osmosis e.g., desalination of sea water, the hollow fibers should have high water-flux as well as high rejection against salt. In hemo-dialysis for a patient suffering from renal failure, metabolic wastes such as urea, uric acid and creatinine should be easily and selectively removed from his blood through the hollow fiber membrance, and simultaneously a well-balanced ultrafiltration property or dewatering property is additionally required. To meet the above requirements, different spinning conditions must be selectively applied to produce the hollow fiber, considering the object and required performances. Various studies have been made to determine appropriate spinning conditions.
When spinning a hollow fiber having a continuously extending hollow portion over its whole length, a spinning solution (dope) is generally extruded from an annular slit to form a sheath solution, and simultaneously a core liquid is introduced into the inside hollow portion of the sheath solution. The core liquids known so far can be classified into two categories.
The first is a liquid having no compatibility with the sheath solution, for example, when a cuprammonium solution of cellulose is extruded as the sheath solution an organic solvent having no compatibility with water is extruded as the core liquid. However, this method has a big disadvantage in that it requires much labor and time to wash the core liquid from the manufactured hollow fiber and the removal of the core liquid is always imperfect due to the incompatibility of the core liquid with water.
The second category is that water or an aqueous solution is used as the core liquid. In this case, interdiffusion between the sheath liquid and the core liquid occurs rapidly. This develops an instantaneous gelation of the sheath solution by the action of a strong coagulating function of water immediately after the extrusion from the annular slit. This phenomenon can be observed even in a dry jet-wet spinning of the hollow fiber. As the result, the spinnability is very poor and the draft ratio is also very small, as shown by the low value of the highest linear speed of the spun filament. That is, the spinning speed is very low, i.e., at most 6 to 15 m/min. The structure of a hollow fiber formed by the rapid coagulation shows considerable effect on a water-flux of the resulting membrane. Such membrane is useful in a sense for ultrafiltration, but it can not be used for the hemo-dialysis because the water-flux is too high, essential ingredients in the blood such as albumin (nutriment source), which have comparatively high molecular weights are undesirably removed, and dewatering performance and waste removability are not well-balanced.
Accordingly, in the conventional method of manufacturing a hollow fiber useful for the hemo-dialysis, a core liquid which is incompatible with the sheath dope showing phase separation has been generally used.
A hollow fiber made of cellulose acetate or regenerated cellulose therefrom has been on the market. So far, the cellulose acetate hollow fiber is manufactured by so-called "semi-melt spinning" or "plasticized melt spinning" in which a plasticizer such as triethylene glycol, triacetin or sulfolane is mixed with cellulose acetate without using a solvent and the mixture is melted and extruded to prepare the sheath part of the hollow fiber. Thus, the polymer melt is extruded to form a hollow fiber in the same way as in melt spinning. Sequently the plasticizer contained in the fiber membrane is removed to give a porous structure thereto. However, this technique allows only a small degree of freedom to impart various porosities. Hence the semi-permeability of the hollow fiber can not be optionally adjusted.
One may have the idea that wet spinning might be applied for the production of cellulose acetate hollow fibers, for example, an acetone solution of cellulose acetate as the sheath dope can be spun in the wet process. In this process advantageously, a coagulating condition of the dope might be variously controlled. Therefore, it might be expected that a hollow fiber having a controlled porosity would be manufactured. Actually, this wet spinning is not suited for the manufacture of hollow fibers for hemodialysis owing to extraordinary solubility characteristics of acetone. Acetone has an excellent capability of dissolving many kinds of solvents, ranging from non-polar substances to polar substances. for Example, it can dissolve, petroleum, hexane, benzene and toluene a well as polar substances such as water and alcohol. Therefore, acetone is compatible with almost all organic and inorganic solvents. When acetone is the solvent component of the spinning solution and an organic solvent is used as a core liquid, the core liquid is compatible with a spinning sheath solution. In this case, the core liquid, which usually has a coagulating function for the spinning solution shows an undesirable ability to coagulate the sheath dope. In more detail, the spinning solution extruded from a nozzle immediately contacts the core liquid, and the core liquid diffuses into the spinning solution to gelate or coagulate the sheath dope because of the easy-miscible property of the acetone component of said dope. Hence, the spinning speed cannot be raised because of the rapid gelation of the spinning solution and a desired hollow fiber for the hemo-dialysis can not be obtained.